БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2016“
BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2016”
Early angiosperm pollen from the Trambesh Formation, Lower Aptian,
Central North Bulgaria
Полен на ранни покритосеменни от Тръмбешката свита, Долен Аптски
подетаж, Централна Северна България
Polina Pavlishina
Полина Павлишина
Sofia University, Department of Geology, Palaeontology and Fossil Fuels, 15 Tzar Osvoboditel Blvd., 1000 Sofia;
Е-mail: [email protected]
Keywords: Early Aptian, angiosperms, palynology, Central North Bulgaria.
Introduction
The appearance of flowering plants and their rapid
radiation during the Early Cretaceous represents a
marked turnover from gymnosperm to angiosperm
dominated ecosystems within a time interval of about
30 Ma. Despite the importance of this significant ecological transformation many aspects regarding the
timing, floral composition, spatial distribution and
radiation are poorly documented. The Barremian–
Aptian time interval appears as particular critical
for understanding the first appearance of the major
angiosperm lineages. At present, detailed and stratigraphically well constrained records documenting the
morphological variability, diversity and abundance of
early angiosperm pollen are relatively rare. For the
northern hemisphere, the locations are mostly situated
along the margins of the Tethyan ocean and the evolving North Atlantic and include the Potomac Group,
United States (Doyle, Robbins, 1977), the Wealden
Group, England (Hughes et al., 1979), the Lusitanian
and Algarve basins, Portugal (Heimhofer et al., 2007).
Most of these early angiosperm records lack independent stratigraphic control due to the absence of
important ammonite or dinocyst markers in the fossil
bearing strata. This fact hampers detailed comparison
between the palynofloras and in addition, their correlation with major climatic and tectonic events during
the Mid-Cretaceous, which had significant influence
on the evolution and rapid diversification of the flowering plants (Heimhofer et al., 2007).
One of the main objectives of this study is to report
and provide documentation of the first finds of early
angiosperm like pollen from the Lower Aptian sediments in the Butovo section, Central North Bulgaria.
The study also aims description of the whole palynological association thus indicating age assessment and
environmental conditions during Early Aptian in the
studied area.
Material studied and geological background
The palynologically productive samples have been obtained from the Trambesh Formation especially from
the Butovo section, Central North Bulgaria. The formation is built by clays, marls and rare, thin sandstone
beds. Recent studies of Ivanov and Idakieva (2013)
provided full description of this section and extensive
biostratigraphical study of the ammonite successions.
The authors recognized the Deshayesites forbesi ammonite Zone, indicating Early Aptian age for the
succession.
Pollen grains of angiosperm – like morphology occur in three of the studied samples, although rare and
in a limited stratigraphic interval. The morphological
study is undertaken under transmitted light microscope and especially SEM and shows their similarities to early angiosperm pollen. The pollen types are
left in open nomenclature until more specimens can be
analyzed and more information becomes available on
their morphological variability.
Palynological results
Besides an exceptionally well preserved ammonite fauna, the Lower Aptian strata in the Butovo section yieldеd
a rich record of palynomorphs including pteridophyte
spores, gymnosperm pollen and early angiosperms
(Fig. 1). Gymnosperms predominate the assemblages.
An overwhelming dominance of the Classo­pollis group
is recorded throughout the section, up to 40% to 50%
of the palynoflora. It often occurs in tetrads due to the
exceptionally preservation conditions. Araucariacites
spp. are also common. Pteridophyte spores account
125
Fig. 1. Palynomorphs from the Trambesh Formation: a, Stellatopollis sp.; b, Classopollis sp.; c, Gleichenidiites sp.;
d, Cicatricosisporites sp.
up to 30% in the assemblages being represented by
Cica­tricosisporites spp., Appendicisporites spp., Bi­kol­
isporites spp., Gleichenidiites spp. Angiosperm pollen
represents a minor constituent of the palynoflora and
accounts for less than 2% in the samples. In terms of
diversity it is represented by monocolpate crotonoid
grains of probably magnoliid and monocotyledonous
affinity attributed to the Stellatopollis genus. These
forms are of particular interest for comparison with
published records. Representatives of the Stellatopollis
group are part of the earliest angiosperm pollen assemblages and have been documented from Upper
Barremian–Lower Aptian interval in southern England,
Portugal and Egypt.
The encountered climate sensitive sporomorphs
have more palaeoecological significance, rather than
stratigraphic value. Their association indicates for
the first time that the studied area was part of the
Southern Laurasian floral province which was characterized by warm temperate to subtropical humid climate. Comparison with published palynofloras from
the other localities in the northern hemisphaere show
strong similarities with regard to floral composition
and especially the timing of first appearances of particular angiosperm pollen forms. Based on the refined
ammonite framework, the results imply that early an126
giosperms played subordinated role in Early Aptian
Bulgarian floras.
Acknowledgements: I would like to express my sincere gratitude to Prof. Marin Ivanov (Sofia Uni­versity)
for the fruitful common field work and help when collecting the samples. Special thanks are due to Prof. Else
Marie Friis (Natural History Museum, Stockholm) for
her valuable scientific expertise for the recognition and
correct systematic assessment of the newly obtained
fossil angiosperm pollen. Financial support from the
Synthesys Program is greatfully acknowledged.
References
Doyle, J. A., E. Robbins. 1977. Angiosperm pollen zonation of
the continental Cretaceous of the Atlantic coastal plain and
its application to deep wells in the Salisbury Embayment. –
Palynology, 1, 43–78.
Heimhofer, U., P. Hochuli, S. Burla, H. Weissert. 2007. New
records of early angiosperm pollen from Portuguese coastal
deposits: Implications for the timing of the early angiosperm radiation. – Rev. Palaeobot. Palynol., 144, 39–76.
Hughes, N., G. Drewry, J. Laing. 1979. Barremian earliest angiosperm pollen. – Palaeontology, 22, 513–535.
Ivanov, M., V. Idakieva. 2013. Lower Aptian ammonite biostratigraphy and potential for further studies of OAE 1a in
Bulgaria. – Cretaceous Research, 39, 47–69.